The present invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for use with boiling water reactors.
A conventional fuel assembly for use with boiling water reactors comprises, as disclosed in JP-A-60-120282, an upper tie plate, a lower tie plate, a plurality of fuel rods fixedly held at their opposite ends by the upper and lower tie plates in the bundle form, a channel box surrounding the bundled fuel rods, and a finger spring attached to the lower tie plate. The finger spring suppresses a leak of cooling water through a gap between the channel box and the lower tie plate. However, a force of the finger spring is exerted on the channel box to push it outwardly. This force accelerates a creep deformation of the channel box at its lower end portion and also spreads a lower portion of the channel box outwardly.
In a fuel assembly disclosed JP-A-54-124183, as with JP-A-60-120282, a leak prevention plate, (corresponding to the finger spring and made of resilient material, is attached to a lower tie plate and is held in contact with a channel box to suppress a leak of cooling water through a gap between the channel box and the lower tie plate. This fuel assembly also raises the similar problem to that in JP-A-60-120282.
With a view of suppressing such a creep deformation of the channel box at its lower end portion, JP-A-61-170692 discloses a structure using no finger spring. With this structure, an inwardly inclined step is provided on the outer peripheral surface of a lower tie plate and a channel box rests on the step. The channel box is closely contacted with the lower tie plate at all times by gravity or its own weight to thereby prevent a leak of the cooling water within the channel box. In addition, because the channel box rests on the inwardly inclined step, an inward component force is produced to act on the lower end portion of the channel box for suppressing an outward deformation of that lower end portion.
Stated otherwise, the fuel assemblies using the finger springs, disclosed in JP-A-60-120282 and JP-A-54-124183, each suffer from the problem that the outward force of the finger spring accelerates an expansion of the channel box at its lower end portion due to creep, and increases a deformation of the channel box.
On the other hand, in the fuel assembly using no finger spring disclosed in JP-A-61-170692, since the lower end of the channel box and the lower tie plate are in close contact with each other at all times, the lower end portion of the channel box undergoes an outward force due to the pressure difference between the inside and outside of the channel box, resulting in accelerated creep. As the lower end portion of the channel box is forced to spread outwardly, the amount of cooling water leaked is necessarily increased in proportion.
In order to suppress the leak flow (rate) of cooling water through the gap between the channel box and the lower tie plate, commonly assigned co-pending U.S. Pat. Application Ser. No. 07,464,151; European Patent Application No. 90300272.3; filed on Jan. 10, 1990) proposed forming a venturi means in that gap without using any finger spring.